This application claims priority of European Patent Application No. 9819136.4, which was filed on Sep. 2, 1998 and European Patent Application No. 98308584.6, which was filed on Oct. 20, 1998.
1. Field of the Invention
This invention relates to packet radio services networks.
2. Description of the Related Art
Standards are being defined for a general packet radio services network (GPRS)
The invention is based on the recognition that if suitably designed a packet services network could carry speech.
To this end, in accordance with the invention there is provided a mobile terminal for communicating with a base station in a packet radio services network, said terminal including a processor for determining one of a plurality of channels for communication between the mobile terminal and the base station; for digitally coding speech to provide speech information; for assembling speech information into speech packets; and for generating channel allocation requests for a channel in which to send speech packets; a radio transmitter for transmitting the requests and the packets to a base station in the network; and a radio receiver for receiving identities of channels allocated by the base station for the mobile terminal to transmit on, said processor being responsive to each received channel allocation to determine that packets are sent on the allocated channel.
In the GPRS since a channel is released when there is no packet to transmit, higher traffic levels can be obtained using the same number of radio channels.
Preferably, if a request to send is not granted, the processor is arranged to discard speech information until a further request is granted. As speech is highly time sensitive, it is better to discard the information than to send the information delayed. The discard produces clipping which, as long as it is not too frequent, is tolerable by the user.
The processor is preferably arranged so that when a request to send is not granted, a further request is delayed by a predetermined period.
The delay is preferably increased if successive requests are not granted.
Following a predetermined maximum delay, the delay is reduced.
The processor is preferably arranged to implement a layered protocol in which each packet is given a header in a subnetwork dependent convergence protocol layer (SNDCP).
Because of the time sensitive nature of speech the header is preferably a RTP/UDP/IP header.
The mobile terminal preferably includes a voice activity detector, and the processor is preferably responsive to detection of voice activity by the voice activity detector, to generate a request for a channel allocation in which to send voice packets, and on receipt of a channel identity, to send the an address header uncompressed on that channel once and subsequently to send packets with compressed headers which do not contain the destination address on the identified channel, until the voice activity detector detects no voice activity.
The processor is preferably arranged to construct packets of an equal number n of frames, the processor being further arranged to implement a logical link layer protocol (LLC) which adds its own LLC header information comprising a service access point identifier defining speech service to each packet, and to divide the total LLC plus SNDCP header into n parts of equal length and to place one header part before each frame in the packet. This provides that every frame in the packet has the same format and allows a common protection strategy to be applied to each frame. The header information can be given an error correcting code. Speech is more error tolerant, however. More important parts of the speech information can be coded in order to identify that there is an error, in which case the frame is discarded. Less important parts of the speech information can be left unprotected.
Thus, in the physical layer, in each frame, the header and the most important bits speech information are preferably coded using a convolutional code, and a subset of the important bits of the speech information are coded using a cyclic redundancy check.
The invention also extends to a base station including a radio receiver for receiving requests from mobile stations to send data packets and requests to send speech packets and operable on a plurality of channels to receive data packets and speech packets; a processor for reserving a predetermined number of said channels for receiving coded speech packets, and for allocating nominating a free one of said predetermined number responsive to a request channel allocation request in which to send a speech packet; and a transmitter for transmitting the allocated channel to the mobile station.
By dynamically managing the number of channels reserved for speech, optimum service can be given to both speech services and to data services given changing respective demands.
The invention also extends to a base station including a radio receiver for receiving requests from mobile stations to send data packets and requests to send speech packets and operable on a plurality of channels to receive data packets and speech packets; a processor for nominating channels for a mobile station to send speech packets and for processing packets in a talk spurt comprising a single destination address header followed by a plurality of speech packets not containing a destination address, for transmission over the network.
The invention further extends to a base station including a radio receiver for receiving requests from mobile stations to send data packets and requests to send speech packets and operable on a plurality of channels to receive data packets and speech packets, a processor for implementing a protocol which recovers network and transport layer headers and logical link layer headers for a packet, from equal parts of each frame in the packet.
The processor may be operative in each frame to correct errors in the header and the most subjectively important bits of the speech information only.